The present inventions are directed to image sensors, particularly CMOS image sensors. Much progress has been made in the field of image sensor design, yet current sensor designs suffer from shortcomings in various aspects. One issue that presents problems for conventional image sensors is limited dynamic range. Dynamic range is the ratio of the maximum measurable signal to the minimal measurable signal. When capturing an image of a scene in which some regions are highly illuminated and others are shadowed, the limited dynamic range of conventional pixel designs used in image sensors leads to a loss of information. Unable to record the full range of tones within the scene, at a given exposure time, a conventional image sensor will typically underexpose the dark regions or overexpose the illuminated regions, such that detail in some portion of the image is lost.
Another shortcoming of conventional image sensor performance is in the arena of low light photography. Low light settings present various problems in digital photography, including color discrimination issues in addition to dynamic range issues. Standard image sensor designs utilize an array of pixels overlaid with an array of color filters, such as a Bayer array or similar means of separating wavelengths to discriminate color within a region of the image. However, the use of the color filter presents a significant disadvantage in a low-light setting because the filter absorbs a substantial proportion of the incoming light, leaving little signal for capture by the pixel. Additionally, the use of a color filter array results in a loss of spatial resolution as the image is reformed. Also, the use of a color filter array causes an increase in image noise due to artifacts inherently produced by color reconstruction processing when the missing colors in each pixel are computationally interpolated.
A known method for extending the light-harvesting capabilities of an image sensor is to remove the color filter array from the pixels, for example as found in the Ricoh Monochrome GRD camera system. However, the resulting images from such sensors are monochromatic (black and white) or otherwise severely lacking in color information. Another method for interpolating color at low light involves the use of “sparse” color filters, wherein color filters are utilized but a substantial portion of the image sensor pixels do not have color filters, for example as embodied in the Truesense™ imaging system. However, such sparse color methodologies require intensive image processing to estimate color, leading to substantial artifacts
Another issue encountered in low-light situations is the presence of highly illuminated areas of the image among very low light areas. For example, within a night scene, there may be local bright spots around lighting fixtures while deeply shadowed areas nearby are very poorly lit. Conventional sensors will often fail to capture image detail in such scenes because the well-lit areas are overexposed (i.e. pixels are saturated) and/or the dark areas are underexposed (i.e. little or no light is captured by the pixel and no image detail is resolvable). Accordingly, there is a need in the art for low-light sensors with very high dynamic range such that differentially illuminated portions of a scene may be effectively imaged.
The present invention addresses these dynamic range and low light color discrimination issues with novel sensor designs and associated methods of using such designs. The present disclosure provides the art with various solutions based upon stacked image sensors, as described below.